1. Field of the Invention
The present invention relates to feedwater supply systems for forced-flow boilers. More particularly, the invention relates to control systems for positive displacement feedwater pumps and a method for supplying feedwater to forced-flow boilers.
2. Description of the Prior Art
Boilers for generating steam can be of the fire-tube type in which the combustion gases are circulated through tubes immersed in a container of water or of the forced-flow type in which water is circulated through tubes which are exposed to the combustion gases. In the former type, the level of water in the container is normally controlled by means of a simple float valve. However, in the latter type, one or more pumps force the water through the tube or tubes at a rate commensurate with the demand for steam. Controlling the rate at which feedwater is provided to such boilers is difficult because of the high pressure (and often high temperatures where condensation from a steam separator is returned to the pump inlet) at which the water must be supplied.
Forced-flow boiler systems for generating steam at a variable rate must include means for controlling the source of heat (i.e., the fuel and air flow to a burner), as well as the water supplied to the heating coil. Controlling the fuel by means of conventional modulating valves and the air by means of conventional dampers is a simple task compared to controlling the amount of water supplied to the boilers. While both variable and constant displacement pumps have been used for supplying the feedwater, constant displacement pumps have an advantage of providing a predetermined output under changing pressure conditions.
A diaphragm-type pump in which an electric motor drives reciprocating pistons within a pump housing, which in turn force hydraulic oil against flexible diaphragms for displacing the water, has been found to be particularly suitable for supplying feedwater to forced flow boilers. Individual pump sections (piston and cylinder) can be disabled through solenoid bypass valves, thereby controlling the pump output in increments related to the number of pump sections, i.e., 3/4, 1/2 or 1/4 output for a four-section pump. Tubular water columns separate the pump head or diaphragms from check valves positioned between an inlet and outlet manifold to keep excessive temperatures from the diaphragms.
Where the amount of water demanded cannot be accommodated by disabling one or more sections of the pump, e.g., 60% of the total pump output, a water bypass valve can be operated to return a portion of the water to the pump inlet. The water bypass valve functions as a modulating valve to accurately supply the required amount of water. Such bypass valves have a tendency to leak and require considerable maintenance because of scale buildup and wear due to solid particles carried by the high temperature water.
As an alternative to the use of water bypass valves, the prior art has used a step control in which the steam output is controlled by turning off (completely or partially) the water, fuel and air flow when the steam pressure reaches one value and turning the fuel, water and air back on when the steam pressure drops to a second value. While such step control systems are less expensive than full modulation control systems, they suffer from several disadvantages.
First, the steam pressure will fluctuate over a considerable range. Second, where the fuel is turned off completely, the combustion chamber must be purged of any residual gases or fuel before it can be refired. While the prepurge period may require only a matter of seconds in a small boiler, i.e., 100-200 horsepower (h.p.), it may require several minutes for a large boiler, i.e., 500 or more h.p. Such a large time delay may result in an excessive drop in steam pressure.
Another alternative to the use of water bypass valves is the use of a hydraulic-actuated diaphragm pump in which the travel of the individual diaphragms (and therefore the quantity of water pumped) is controlled by varying the quantity of hydraulic fluid delivered to the diaphragms. A pump of this type is described in U.S. Pat. No. 3,972,654. While such pumps have been successful in accurately controlling the delivery of feedwater and eliminating the leakage problem of water bypass valves, they are expensive to manufacture.
These and other disadvantages of the prior art feedwater control systems for forced-flow boilers have been overcome by the present invention.